Opal glasses have been known to the glass art for many years. These glasses have commonly been classified into two broad categories, viz., (1) spontaneous opal glasses, and (2) thermally opacifiable or reheatable opal glasses. The first category encompasses those compositions which develop opacity as the molten batch is cooled to a glass article, this opacity resulting from the growth of crystals in the glass or from some other type of phase separation phenomenon taking place within the glass. In the second category of glasses, little or no opacity is achieved when the molten batch is cooled. Opacity will develop, however, when the glass bodies are exposed to temperatures in the vicinity of or somewhat above the annealing point of the glass. Here, again, crystal growth or some other phase separation phenomenon produces the desired opacity. The crystal content developed is generally quite small, most usually less than 10% by volume, so that the overall physical characteristics of the glass, other than optical transmission, are affected only slightly, if at all.
Whereas such opal glasses can and have been marketed commercially when in the pristine state, i.e., with no glaze or other surface decorating medium applied thereto, consumer interest in such has been limited. This has been especially true where these materials have been utilized in such applications as culinary and table ware. There the customer demands a product which not only provides serviceable utility but is also aesthetically pleasing.
A dense, uniformly milky-white appearance has been deemed to constitute a most desirable attribute of opal glasses. Such glasses permit the manufacture of thin-walled and, therefore, lightweight articles, but which will still exhibit good opacity. However, such glasses have been subject to a serious problem termed "flask mark", when molded articles such as culinary and table ware have been produced therefrom. This defect is evidenced by an area of differential opacity near the bottom of such articles and is a frequent source of rejection for the ware. Also, because of their compositions and consequent physical properties, considerable difficulty has been experienced in devising applicable fired-on decorating materials.
Decorative enamels have been employed extensively in the past to impart color and/or gloss to opal glass articles. Such enamels are applied to the surface of the glass in frit form and heated to a sufficiently high temperature to cause the enamel to fuse and flow smoothly over the surface, thereby attaining a homogeneous appearance and proper adherence to the glass substrate. Hence, the enamel must have a flow point which is at a low enough temperature to avoid thermal deformation of the opal glass substrate. Also, the enamel should have a coefficient of thermal expansion equivalent to, and preferably lower than that of, the opal glass to inhibit crazing or spalling and to place the enamel layer in compression after the firing step. Finally, lead and/or cadmium have commonly been included in enamels to provide a high refractive index and a low melting point thereto. However, those elements are extremely toxic which demands that the resistance of the enamel to chemical attack, e.g., acids, alkalies, and sulfides, be sufficiently great that release of those elements is extremely low. Thus, for food contact surfaces, a standard for lead release has been set as not exceeding about 2 .mu.g/cm.sup.2 of contact surface and that for cadmium, when present, as not exceeding about 0.2 .mu.g/cm.sup.2 of contact surface. For non-food contact surfaces, these standards are 50 .mu.g/cm.sup.2 and 5 .mu.g/cm.sup.2, respectively. The enamel also evidences good resistance to attack by household dishwasher detergent.
Opal glasses wherein the opacity mechanism is the presence of alkali metal or alkaline earth metal fluoride crystals have been known to the art for many years. For example, U.S. Pat. No. 2,224,469 describes spontaneous opal glasses containing sodium and/or potassium fluoride crystals and having compositions which are preferably free of B.sub.2 O.sub.3 and consist essentially, in weight percent, of 13-16% alkali metal oxide, 5-9% Al.sub.2 O.sub.3, 65-80% SiO.sub.2, and at least 3% of analytically determined fluorine. As a further example, U.S. Pat. No. 2,921,860 discloses spontaneous opal glasses containing sodium fluoride crystals and having compositions consisting essentially, in weight percent, of 0.5-3% Li.sub.2 O, 6-19.5% Na.sub.2 O, the total alkali metal content being 12-20%, 2-12% Al.sub.2 O.sub.3, 55-75% SiO.sub.2, and 5-9% F. As a still further example, British Patent No. 1,289,185 discloses spontaneous opal glasses containing lithium fluoride and sodium fluoride and/or potassium fluoride crystals and having compositions consisting essentially, in weight percent, of 0.5-5% Li.sub.2 O, 2-18% Na.sub.2 O and/or K.sub.2 O, 1.5-12% Al.sub.2 O.sub.3, 60-80% SiO.sub.2, and 2-6% F.
Each of those patents describes spontaneous opal glasses which can be marketed commercially but there has been the desire to develop products demonstrating the opacity character equivalent to fine china while, at the same time, meeting stringent manufacturing and use requirements and avoiding the occurrence of flash mark during molding.
As was observed above, enamels for decorating ceramic bodies have been employed for many years. In general, the art has considered enamels to consist of two parts: the "flux" or glassy portion and the pigment. Pigments have been added as opacifiers and to provide coloring, where desired. In the usual practice, the pigment will comprise a minor portion of the enamel, normally less than about 20%, so it is the flux portion which customarily governs the overall properties of the enamel. And, therefore, it is the flux portion which has formed the source of considerable research to provide compatibility with various substrates while maintaining chemical durability, gloss, etc.
U.S. Pat. No. 2,225,162 is an early example of enamels which could be applied to glass substrates. Those enamels were stated to exhibit excellent resistance to attack by sulfur-containing materials and were formed from compositions consisting essentially, in weight percent, of 1-4% Li.sub.2 O, 1-6% TiO.sub.2, 30-60% PbO, 30-50% SiO.sub.2, and 0-10% B.sub.2 O.sub.3. The preferred compositions contained equimolar amounts of Li.sub.2 O and Na.sub.2 O. The exceptional resistance to sulfide attack was declared to be due to the joint presence of Li.sub.2 O and TiO.sub.2.
U.S. Pat. No. 2,278,868 provides glazes statedly demonstrating excellent resistance to the action of alkalies. Such glazes consisted essentially, in weight percent, of 2-15% B.sub.2 O.sub.3, 0.5-10% R.sub.2 O, wherein R.sub.2 O consists of 0-3% Li.sub.2 O, 0-5% Na.sub.2 O, and 0-8% K.sub.2 O, 1-10% ZrO.sub.2, 40-60% PbO, and 20-30% SiO.sub.2. The presence of ZrO.sub.2 is observed to impart the exceptional resistance to alkalies. The disclosure notes that, whereas Li.sub.2 O is the preferred R.sub.2 O component for maximum resistance to alkalies, its presence leads to incompatibility between the expansion of the glass substrate and the glaze. Therefore, Na.sub.2 O is the preferred R.sub.2 O.
U.S. Pat. No. 2,312,788 involves enamels statedly demonstrating high resistance to acids, alkalies, and sulphides consisting essentially, in weight percent, of about 1.5% Li.sub.2 O, 3% TiO.sub.2, 6% ZrO.sub.2, 4% Na.sub.2 O, 3% CdO, 7.5% B.sub.2 O.sub.3, 49% PbO, and 25% SiO.sub.2.
U.S. Pat. No. 2,356,316 describes enamels exhibiting improved alkali resistance consisting essentially, in weight percent, of about 3-7% Na.sub.2 O, 3-12% B.sub.2 O.sub.3, 5-15% BaO + ZrO.sub.2, 40-60% PbO, and 22-32% SiO.sub.2. Where acid resistance is also desired, up to 5% TiO.sub.2 can be added. The inclusion of BaO or ZrO.sub.2 alone will improve resistance to alkali attack, but the combination of the two is even more beneficial and the final gloss is better.
U.S. Pat. No. 3,404,027 discloses enamels for glassware displaying a satin finish composed of a flux consisting essentially, in weight percent, of 0.5-4% Na.sub.2 O, 2-10% B.sub.2 O.sub.3, 0-3% Li.sub.2 O, 0-5% ZrO.sub.2, 0-8% TiO.sub.2, 50-65% PbO, and 25-35% SiO.sub.2 which is combined with mill additions of 0.1-4% Cb.sub.2 O.sub.5 and 10-30% TiO.sub.2 and/or Al.sub.2 O.sub.3.
Nevertheless, as has been explained above, the use of a particular enamel is governed by its compatibility with the substrate to which it is applied. Of extreme importance are the flowpoint and coefficient of thermal expansion of the enamel in relation to the softening point and expansion of the substrate. Therefore, an enamel of singularly-defined properties is required for the desired opal glass referred to above having an opacity character equivalent to fine china and meeting stringent manufacturing and use requirements while avoiding the problem of flash mark during molding.